ZIP

Revision as of 19:32, 16 September 2013

Not to be confused with Zip disk, an unrelated disk cartridge unit.

ZIP is one of the most popular file compression formats. It was created in 1989 as the native format of the PKZIP program, which was introduced by Phil Katz in the wake of a lawsuit (which he lost) against him by the makers of the then-popular ARC program (and file format) for copyright and trademark infringement in an earlier program PKARC which had been file-compatible with ARC. This resulted in Katz creating a new file format, which rapidly overtook ARC in popularity (to a large extent because of BBS sysops, then the primary users of such compression, resenting the lawsuit). Many programs have been released for a variety of operating systems to compress and decompress ZIP files, and native support for the format is built into several popular operating systems.

ZIP implementations vary in their support for features in the specification from PKWARE (http://www.pkware.com/documents/casestudies/APPNOTE.TXT), particularly features added since version 2 (1993), some of which are protected by patents and require licensing. Many implementations limit the use of compression to the DEFLATE algorithm, introduced with version 2. Extensions incorporated into the specification that have been widely adopted are: long filenames; large files (using a technique known as ZIP64); and filenames in UTF-8. In 2011 work began on an interoperable subset of the latest APPNOTE.TXT with the intention of publication as ISO/IEC 21320-1, Document Container File -- Part 1: Core. As of November 2012, a discussion draft is available at http://www.itscj.ipsj.or.jp/sc34/open/1855.pdf. Designed to promote interoperable implementations, the draft ISO/IEC 21320-1 prohibits compression other than using DEFLATE, segmentation or multiple volumes, and features that are subject to patents.

While .zip is the usual file extension, ZIP-formatted files can be found with many other extensions since a number of other file formats use ZIP compression but store their files in application-specific extensions.

Identifiers

• File extension: .ZIP
• MIME type (Internet media type): application/zip
• Uniform Type Identifier (Apple): com.pkware.zip-archive
• PUID (PRONOM): x-fmt/263

References

• ZIP (file format): Wikipedia
• APPNOTE from PKWARE (latest version of formal spec)
• APPNOTE Archives from PKWARE (selected versions since 4.5.0 from 2001)
• IANA registration for application/zip in July 1993 (corresponds to version 2 of APPNOTE.TXT)
• Documentation from Info-ZIP (Includes Info-ZIP variants on APPNOTE.TXT dated from 1996 to 2004, specifications used as the basis for various open-source tools)
• ZIP File Format (PKWARE), from Library of Congress resource on Sustainability of Digital Formats
• ZIP Format summary at PRONOM
• November 2012 working draft of ISO/IEC WD 21320-1, Document Container File -- Part 1: Core Intended as restricted subset of APPNOTE 6.3.3 designed to promote interoperability.
• February 2013 committee draft of ISO/IEC CD 21320-1, Document Container File -- Part 1: Core Essentially the same as November 2012 working draft except that it mandates use of the UTF-8 indicator.
• Archive format info, including ZIP (from 1989, when ZIP was newly released)